The present invention relates to a driving apparatus for a retaining frame of an object to be sewed in an automatic sewing machine and, more particularly, an apparatus for driving a tabouret or embroidery frame retaining an object-to-be-sewed to move in the X axis direction or the Y axis direction relative to the position of a sewing needle so as to perform the optional sewing operation in an automatic sewing machine for stitching an embroidery pattern on the object.
More specifically, the present invention pertains to the frame positioning device wherein a stepping motor assembly is employed for each of two drive mechanism for respectively moving the frame in an X-axis direction and in a Y-axis direction perpendicular to the X-axis direction to bring the frame to a predetermined coordinate position. The stepping motor assembly itself may be comprised of a plurality of pulse responsive stepping motors, wherein either respective stators or rotors remain aligned in phase with each other, and the other associated rotors or stators may be angularly offset at a predetermined angle relative to each other on a machine drive shaft.
Conventionally, as shown in an automatic embroidering machine of FIG. 1, an automatic sewing machine is composed of a sewing machine body provided with a driving mechanism 2 for vertically moving an embroidering needle 1 to perform the embroidering operation, a frame 4 for retaining an object 3 to be embroidered thereon, a X-axis driving mechanism 5 for moving the retaining frame 4 in the X-axis direction, a Y-axis driving mechanism 6 for moving the retaining frame 4 in the Y-axis direction, and a control system for controlling the operations of mechanisms 2, 5 and 6 in such a manner that the X-axis numerical data and Y-axis numerical data for respectively moving the retaining frame 4 in the X-axis direction and the Y-axis direction, and the controlling data for controlling the sewing machine body 2 are sequentially read out to a memory circuit 9 by a tape reading apparatus 8 from a numerical data tape 7 such as punched tape or the like, and the data is temporarily stored within the memory circuit 9. A machine driving motor 11 of the driving mechanism 2 for reciprocatingly moving the embroidering needle 1 is adapted to be driven, through a machine drive controlling circuit 10, in accordance with the controlling data outputted from the memory circuit 9. Pulses outputted from a pulse generator 12, which is composed of a crystal oscillating circuit or the like for generating the pulses of a constant frequency, are inputted to a X-axis pulse converting circuit 13 and a Y-axis pulse converting circuit 14, each of which is composed of a gate circuit or the like, and such circuits are controlled by the X-axis numerical data (X) and the Y-axis numerical data (Y) outputted from the memory circuit 9. Pulses outputted from the pulse generator 12 are triggered to transmit the X-axis numerical data (X) and the Y-axis numerical data (Y) from the X-axis pulse converting circuit 13 and the Y-axis pulse converting circuit 14 to a X-axis driving circuit 15 and a Y-axis driving circuit 16, respectively, as the pulse row of numerical data number. The X-axis driving circuit 15 and Y-axis driving circuit 16 respectively drive an X-axis stepping motor Mx of the Y-axis driving mechanism 5 and a Y-axis stepping motor My of the Y-axis driving mechanism 6. The X-axis driving mechanism 5 and the Y-axis driving mechanism 6 move the retaining frame 4 respectively in the X-axis direction and the Y-axis direction to thereby perform the optional embroidering operation.
However, as apparent from the above description in the automatic embroidering machine, the moving speed of the retaining frame 4 for the object-to-be-embroidered 3 is regulated by the frequency of a reference pulses outputted by the pulse generator 12. The numerical data of the memory circuit 9 for determining the motion amount of the retaining frame 4 in the X-axis direction and the Y-axis direction vary principally from 2.sup.0 to (2.sup.n -1) in accordance with the instruction contents of the numerical tape 7. Accordingly, the moving time of the retaining frame 4 to be driven every time by the stepping motors Mx and My is varied in proportion to the numerical data by the tape instructions. Thus, the frequency of the reference pulses of the pulse generator 12 is normally set to a maximum limit value in order to be able to transmit the instructions at the maximum numerical value of the tape 7. As soon as the embroidering needle 1 leaves the object-to-be-embroidered 3, the retaining frame 4 begins to move and, then, a given amount of the motion of the retaining frame 4 is set to be normally completed, before the embroidering needle 1 is thrust into the object 3 again, even if the maximum moving time of the retaining frame 4 is required by the data of the maximum numerical value. If it is assumed that the frequency of the reference pulse is f(0) and the time t(0) required for reciprocating the embroidering needle 1 to leave from and to thrust into the object 3 at every cycle, i.e. the allowable length of time with respect to the motion of the retaining frame 4, then it is necessary to normally maintain the relationship: 1/f(0).ltoreq.t(0) in order to prevent the motion of the retaining frame 4 from interfering with the embroidering needle 1. Although the allowable time length t(0) depends upon the rotating speed of the mechanical system such as the machine driving motor 11 or the like and the frequency f(0) of the reference pulse is artificially set in the synchronous range, considering the above conditions of the retaining frame 4 and embroidering needle 1, in the pulse generator 12, both values are independent of each other. Thus, when the numerical data instructed by the tape 7 is set at one half the maximum numerical value, the retaining frame 4 is driven to complete its motion in the first half of the allowable time length t(0) and waits for the re-thrust of the embroidering needle 1, the retaining frame 4 being in an inoperative position during the second half of the allowable time. Since most of the numerical data of the tape instructions used for the normal operation are smaller than the maximum value and remain, in average value, from 60% to 70% of the maximum value, in the automatic embroidering machine, a waiting time of from 30% to 40% is usually wasted, thus resulting in a decrease in the top limit of the operation speed of the mechanical system. Also, when the rotating speed of the sewing machine has been increased due to some reasons, the allowable motion time changes to a value t(1) which is less than t(0), and, unless the correction of proper frequency f(0) is otherwise applied, the motion time of the retaining frame 4 depending upon the frequency f(0), which was pre-determined in accordance with the speed of the mechanical system before the changing is extended to 1/t(0)&gt;t(1), when the numerical data is maximum in value or is close thereto. As a result, the allowable motion time becomes insufficient and the embroidering needle 1 might be broken or the object 3 might be torn off. In addition, the embroidering needle 1 may be left within the object 3, the retaining frame 4 started to move so as to interfere with the embroidering needle 1, and the given motion of the retaining frame 4 not completed even in the re-thrust of the embroidering needle 1. Also, after the thrust, the remaining movement of the frame is performed. Even when the user decreased the rotational speed of the sewing machine due to sewing machine thread cutting or to improve the operational quality, the moving speed of the frame did not change, thus being unable to take advantage of the resultant speed change.